The invention of photographic film in the last century began the process of recording a spatial light distribution, generally with the end goal of forming an "image" of an external "object." It was soon recognized that only a small fraction of light incident upon film was actually being converted into a useful exposure. This concept was ultimately embodied by the term sensitivity.
The advent of electronic means of forming images, first realized by scanned photo-electric converters, then by scanning tubes, and most recently by charge coupled devices and similar wafer-scale devices, offered a multitude of improvements to the photographic film process, the improvement of sensitivity being among them, especially in the case of charge coupled devices.
An ideal array-based detector is one which can determine the exact arrival time, the exact detection location, and the exact wavelength of each and every photon which falls upon a detection device. This ideal is generally far from being realized, and indeed, is not actually required for a vast majority of applications. But proximities to this ideal are helpful for many applications.
One such area is precisely the improvement of sensitivity, especially in relation to the natural noise processes of light detection and measurement. Scientific grade charge coupled devices, intensified cameras, and photon counting cameras are three rather fuzzily defined classes of instruments which attempt to accurately measure lower and lower counts of photons (dimmer and dimmer objects), relative to natural and instrumental noise sources. Scientific grade CCDs generally approach this goal by lowering the "read-noise" of collected photo-charge, where generally a single sensed photon gives rise to a single photo-electron. Even in the best of devices, however, the read noise is quite appreciable relative to distinguishing a single photon event.
Intensified cameras and so-called photon counting devices generally effect an amplification of a single photo-electron into a multitude, or "gain," in electrons, thereby lowering the effective read noise of a device by the very fact of amplifying the signal. Though this approach has proven quite valuable in many applications, it does have its drawbacks, such as cost, lowered quantum efficiency, limited dynamic range and reliability, among others. In photon-counting devices, cost and complexity remain a problem, and devices are often limited in the number of photons per second that can be reliably measured (dynamic range), along with decreases in quantum efficiency, i.e., sensitivity. All prior art devices of these two classes known to the present inventors involve a "staged" approach toward amplification, generally consisting of a detector separate from the amplification mechanism.
Avalanche photo-diode devices have evolved as single channel detectors (i.e. non-array based detectors) in order to integrate onto a single semi-conductor device the process of electron amplification AND signal reading or measurement. Recently, design ideas and certain device realizations have been published which outline basic wafer scale integrations of avalanche semi-conductor principles with prior art array detector principles. See, for example, Komobuchi, et al, "A Novel High-Gain Image Sensor Cell Based on Si p-n APD in Charge Storage Mode Operation," IEEE Trans. on Electron Devices, 37:8, August, 1990, pp. 1861-1868, and Hynecek, "CCM--A New Low-Noise Charge Carrier Multiplier Suitable for Detection of Charge in Small Pixel CCD Image Sensors," IEEE Trans. on Electron Devices, 39:8, August, 1992, pp. 1972-1975. A common characteristic of these prior arts and that of the hereafter disclosed invention is that a single photoelectron is effectively amplified into a (generally selectable) multitude of electrons in the device domain (pre-output amplified) through the process of the avalanche photo-diode. There are vast differences, however, in the implementation of this basic idea between that suggested by the published literature and that of the present invention.
One embodiment of the present invention contemplates the separate fabrication of an avalanche photo-diode array and a conventional charge coupled device, and their subsequent alignment and fusing into a single wafer scale device. Certain basic modifications to each prior art technology must be made in order to ensure that the fabricated devices are not merely laboratory curiosities, but are capable of true commercial quality and yield. The inventors refer to this as wafer-scale integration of photo-electron amplification and detection, in distinction to "staged" separate elements as in prior art intensified cameras usually characterized by the use of multi-channel plates, and in distinction to the miniaturized "packing" of prior art avalanche photo-diodes either in storage mode or in direct X-Y addressable read out schemes.
Many potential advantages fall out from such an arrangement, including cost, high quantum efficiency, variable on-chip gain, improved thermal noise characteristics, among many others. Component devices can be independently tested prior to fusing, increasing overall yields. Likewise, the "division of labor" between photo-sensing and photoelectron transport can be effected, allowing for independent optimization of these two distinct functions. This alone can help increase individual yield of the two constituent devices. As with some of the prior art, the amplifying process is entirely at the wafer scale and uses the inherently low noise process of avalanche photo-diodes. The separate fabrication of the sensing stage allows for the use of prior art backside thinning processes, which is critical for achieving very high quantum efficiencies across a broad range of wavelengths. The use of the CCD readout mechanism ensures optimal noise performance, reducing demands on the required gain in the avalanche stage, which ultimately improves the statistics of "true counts" versus "false counts," improves the dynamic range, and brings us several steps closer to pure shot-noise-limited detectors. Finally and quite importantly, normal engineering evolution of these devices will migrate toward smaller and smaller pixel sizes, which, when combined with their potential for low cost fabrication, lend themselves to the current drive toward electronic detectors for HDTV, electronic cinema, radiology and high definition photography. For these types of applications, the mere presence of low inherent noise gain is sufficient advantage and they generally have no use for the concept of `photon counting.